Double blind administration of a solution of norepinephrine or dopamine contained either in vials or syringes according to the ICU's protocol (see appendices I and II for vial and syringe preparations). Each solution will be labeled with its randomly allocated number and will be prepared by the nurses in charge. The rate of administration of the blind solution will be determined according to a scale and using the estimated body weight. The recommended steps for increasing or decreasing the doses of the blinded solution are calculated to be equivalent to 2 µg/kg.min for dopamine and 0.02 mcg/kg.min for norepinephrine, the investigator being free to use multiples of this increment in case of emergency. The maximal dose with the 2 agents will be 20 µg/kg.min for dopamine and 0.19 mcg/kg.min for norepinephrine (these doses have been shown to have similar effects on mean arterial blood pressure 4).